DISI.iterator() should never return null. [LUCENE-5117]

If you have a Filter, you have to check for null twice: Filter.getDocIDSet() can return a null DocIDSet, and then DocIDSet.iterator() can return a null iterator.

There is no reason for this: I think iterator() should never return null (consistent with terms/postings apis).

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Migrated from [LUCENE-5117](https://issues.apache.org/jira/browse/LUCENE-5117) by Robert Muir (@rmuir), updated Oct 23 2014

Start by locating DISI.iterator(), Filter.getDocIDSet(), and DocIDSet.iterator(), then inspect their implementations and existing tests. Confirm how null DocIDSets and iterators are handled across the search APIs; done means iterator() no longer returns null and the relevant behavior is covered by tests.

Written by the indexing model from the issue text.